(A) Anterior vena cave from head. (B) Aorta to body and head. (C) Pulmonary artery to lungs. (D) Pulmonary veins from lungs. (E) Left atrium. (F) Left ventricle. (G) Right ventricle. (H) Right atrium. (J) Posterior vena cava from body

The heart is the centre of the circulatory system acting as a pump circulating the blood through arteries and veins. Blood collects in the left auricle (E above). As the left auricle (E above) fills up an impulse is triggered that results in the first chamber contracting and pushing the blood into the left ventricle (F above 6 below). The ventricle is the main pumping chamber of the heart. When this is full it contracts and pumps the oxygenated blood into the main artery (dorsal aorta) and around the whole body. In the tissues, blood flows into capillaries, allowing transfer of nutrients and waste products. Waste products are then carried away in veins to relevant organs that dispose of them. The veins lead into the anterior (A above 9 below) and posterior vena cava (J above) and so to the right ventricle (G above).

The right ventricle (G above) contracts and sends the blood into the pulmonary artery (C above or 2 below) where it is taken to the lungs to dispose of carbon dioxide and pick up oxygen. The blood leaves the lungs via the pulmonary vein (D above 3 below) and goes back to the left ventricle (F above 6 below) where the cycle start again. The two sides of the heart work in synchrony. The two auricles and ventricles contract in pairs.

(1) Aorta. (2) Pulmonary artery. (3) Pulmonary veins. (4) Diaphram. (5) Left atrium. (6) Left ventricle. (7) Coronary blood vessels. (8) Right atrium. (9) Anterior vena cava.